The invention relates to a method and an apparatus for adjusting and maintaining the absolute seating surface height of a spring-action passenger seat, independent of interference factors, particularly in a motor vehicle.
Spring-action passenger seats, which rest or are installed on a non-fixed base, are affected by a plurality of external interference factors. This is particularly true, for example, for driver's seats in trucks, buses, construction equipment, rail vehicles and many others. Here, the seating surface height as set by the driver in accordance with his individual height is supposed to be maintained, on an average over time, in spite of external influences and the temporary effect of the spring action of the seat.
In this connection, there is the problem, for one thing, that persons with approximately the same height can have different weights. A heavy person will therefore "sink into" the seat to a greater degree, even in the stationary case, i.e. will put greater stress on the seat springs, than a person of the same height with less weight. Therefore one task consists of ensuring that the seating surface height, which is essentially dependent on the person's height, is maintained, at least independent of the weight of the person resting on the seat.
In practical situations, however, a plurality of other, particularly dynamic interference factors acts on a seat which has means to adjust the seat height and seat springs, and these influence the seating surface height that really comes about. The surface on which a seat is placed in a truck, for example, is subject to horizontal shocks of varying amplitude and frequency, which may be caused by unevenness of the road surface and vibrations caused by the motor, and which can also be dependent on the load of the motor vehicle. Such influences provoke at least temporary, in some circumstances even permanent deviations of the seating surface height from the value that has been set as optimal for the person's height.
Other influence factors which occur are wear, the lubrication condition and the level of dirtiness of the seat mechanism. Thus, the mechanism can become more difficult to move after longer use. The lift of the seat springs in leveling out road bumps, for example, becomes less, and greater internal friction and adhesion forces cause an earlier delayed response of the seat springs. In case of great wear, the reverse can also occur, namely that the mechanism loses its internal tension and demonstrates significant play. In this case, too, increasing permanent deviations from the desired seating surface height can occur with extended use of a seat.
Finally, climatic influences, particularly humidity and ambient temperature, also affect the functioning of the seat mechanism. For example, during the winter, when a motor vehicle is cold-started, the case can occur that the seat mechanism is temporarily stiff, but the seating surface height slowly drops with an increased warming of the motor vehicle interior, e.g. due to sun radiation, and takes a lower stationary level. In spite of the fact that the seating surface height was originally set at the optimum for the person's height, a permanent change in the seating surface height occurs over time, during operation. Finally, changing humidity conditions, for example in case of rain or snow, can also have an effect on the proper functioning of the seat mechanism.
From DE 27 36 242 A1, DE 33 12 732 A1 and U.S. Pat. No. 4,589,620, spring-action passenger seats are already known, in which a predetermined reference position of the seat is maintained, independent of the weight of the person resting on them. The known embodiments are based on the common principle that at least two comparator-like detector elements which are fixed in place, arranged one above the other, are present, and detect impermissibly high deviations of the seating surface height from the reference position. When a binary response signal of the upper or lower detector element occurs, the seat is lowered or raised, respectively. The method of functioning of the known embodiment therefore essentially corresponds to that of a two-point regulator. These arrangements have the disadvantage that the distance between the two limit switches, which lie one above the other, has to be selected relatively large. However, this impairs the ability of the seat springs to absorb shocks, for one thing. For another, the predetermined reference position can only be achieved with a relatively great inaccuracy, which depends on the distance between the limit switches that lie one above the other.